1. Field of the Invention
This invention relates to an apparatus for treating substrates with a liquid, for example, to such apparatus which chemically treats or cleans a substrate with chemical solutions or water.
2. Description of the Prior Art
Conventionally, thin plates or substrates (which are referred to simply as "substrates" hereinafter), such as semiconductor wafers, glass masks, reticles, compact disks or laser disks, have been kept vertical to undergo a drying process after being treated or cleaned with chemical solutions or deionized water (D. I. water).
In such conventional drying methods, there is a method wherein after a substrate 3 (see FIGS. 12-15) which is contained vertically in a recess 2 in a cassette 1 has been cleaned, liquid adhering to its surface is removed by centrifugal force in a high speed spinning machine.
However, problems of this method are that the substrate itself may be cracked by the high speed spinning, and that dust and the like from bearings of the spinning machine may adhere to the substrate. These are factors which can cause defects in patterns formed on the substrate.
For other methods to solve these problems, there are drying by blowing hot air; utilizing vapor of an organic solvent; utilizing surface tension of a liquid, and so on, but there remains problems shown below.
(1) After drying by blowing hot air, evaporation marks of the water drops may remain. PA0 (2) Because the method of removing water by exposure to the vapor of an organic solvent requires a large quantity of the organic solvent, and in addition solvent is heated to evaporate, there is constant danger of catching fire and explosion. PA0 (3) In one method, water is removed from a surface utilizing the surface tension of the water as the substrate is slowly lifted from it. In this method, D. I. water which is supplied to a tank and which conducts the cleaning overflows from the tank. However, as a result of the inventors examination, the following problems may arise in this method.
With particular reference now to FIG. 14, a sectional view showing the state wherein the substrate 3 contained in the cassette 1 is being immersed into D. I. water 30 of a tank 9 according to above describe method (3), the D. I. water 30 is supplied from the lower part of tank 9 to conduct cleaning, overflowing from top edges of side walls 10 of the tank 9. If the water 30 is not allowed to overflow uniformly from the top of the all side walls 10, problems may arise. Thus, the edge of the side walls 10 must be precisely horizontal in order to make the water 30 overflow uniformly. It is very difficult to make the tank 9 with such precision. Supplying a large amount of the water would solve the problem, but this raises the cleaning cost. When the substrate 3 is immersed into and lifted from the water 30, dust, for example, created by the vertical lift mechanism (not shown) which moves a cassette conveying arm 38 holding, originally adhered to the cassette and the substrate themselves, or existing in the water, might float to the surface of the water 30. At this time, if the overflow of the D. I. water 30 from the tank 9 is not uniform from the top of the four side walls 10, the flow of the water 30 becomes inactive and stagnant in some part of the tank 9.
FIGS. 15 and 16, enlarged sectional views of the apparatus of FIG. 14, show movement of the dust to a place where the flow of the D. I. water becomes inactive and stagnate. In these Figures, only a piece of one substrate 3 is shown, although there may actually be contained, for example, a large number of substrates cassette.
FIG. 15 particularly shows the state of the substrate 3 as it is being lowered to be immersed into the water 30. As previously described, if, for example, dust caused by the vertical lift mechanism falls onto the water 30, this dust 8A, shown floating on liquid surface 30a, is not flowed out of the tank in the place where the water is stagnant, but enters the water 30 and may adhere to the substrate 3. Dust 8B adhering to the substrate 3 might not be removed even by the upward flow of the water, because the flow of the water is inactive in an area of a minute width adjacent to the substrate 3.
As shown in FIG. 16, as the substrate 3 is lifted from the water, after the substrate 3 has been cleaned for a predetermined time, the dust 8A previously described floating on the surface 30a of the water 30 and which is not flowed out of the tank, may adhere to the substrate 3 when it is lifted, led by the water which makes the substrate 3 wet. Therefore, in addition to above described dust 8B, the substrate 3 is further contaminated by dust 8C which adheres thereto when it is lifted.
Dust particles 8B and 8C remain adhered to the substrate 3 even after it is dried, which could lead to substrate contamination or defects in patterns formed later on the substrate.
These problems arise also in chemical treatment of substrates or other objects, besides in water cleaning treatment.